This sounds good since in
pijul, patches are known by their hashes, for the most part (if not ONLY). For example:
# Dependencies [a] ... [b] ... [*] ...
I think what you are talking about (whether we call it topics or whatever else) is a good idea.
Mercurial’s evolve extension has a very interesting feature: topics. In short, topics categorizes commits by adding a shared tag, which can be leveraged by various commands. For example, it is simple to only log, bisect or prune (equivalent to unrecord) commits within a certain topic. Also, it adds semantic meaning to commit messages and it is a powerful model to handle dependend commits. I think that it is a very useful feature and that I only scratched the surface of what could be done with that.
If Pijul should gain a similar feature, I think it should have these properties: